You've asked a good question - one that's a potential concern to all RPi users. I'm still working through this - I may not have a complete answer now, but I'll post what I've learned now, and update when I gather "the rest of the story". Feedback, corrections and comments are welcomed.
Some clarification on semantics is always useful when discussing Debian's Advanced Packaging Tool (APT):
- APT is a generic term referring to the collection of tools used for package management.
apt-get is a 'command-line tool for handling packages, and may be considered the user's "back-end"'apt is 'a high-level commandline interface for the package management system. It is intended as an end user interface and enables some options better suited for interactive usage by default compared to more specialized APT tools like apt-get(8) and apt-cache(8)'
This was not an "error" per se
It's simply information - some feedback from apt letting you know what happened as it attempted the update you requested. apt-get update ran successfully to completion, but it found some things that you should know. It declined to update because it found discrepancies, and it wants you to acknowledge the discrepancy before it performs the update.
Why did apt-get not perform the normal update process?
A lot of details behind this, but I'll summarize it as follows:
RPi OS is a derivative of Debian
Debian officially released bullseye on Aug 14, 2021
As is typical, the official release of RPi's new version lags the release date of Debian by 2-4 months. This because some aspects of the RPi release are dependent upon having a stable Debian release.
This "gap" between the Debian release and the RPi release creates discrepancies in the labels used to identify and segregate branches of the repositories. In particular, the stable label is now assigned to the newer, leading Debian bullseye version. Consequently, the buster branch must surrender the stable label; the RPi and/or Debian maintainers have decided to change the label to oldstable.
apt doesn't know of this change. It wants to follow stable, but stable is now in bullseye - not in buster. Rather than panic, it simply throws the decision over to the user... "what do you want to do, boss?"
What should I do?
You have many options - here are a few to consider:
You can ignore it, and forego updates for a while. This isn't as bad as it sounds... with RPi maintainers hard at work on the bullseye release, there is very little time for changes to buster. To learn when your last upgrade was performed, run less /var/log/apt/history.log. If it's been since Aug 14, you're likely not missing many updates.
You could go ahead and upgrade to a pre-release version of bullseye using one of RonR's scripts.
You can continue following buster. Debian releases are maintained for 3 years, and come under Long-Term-Support for an additional 2 years, so buster will be around for a while. And of course you may also choose to follow buster only until bullseye is released for RPi.
Under this option, you will need to manipulate apt-get to continue tracking buster. One way to do this is to employ the option --allow-releaseinfo-change; see man apt-get for further details. N.B. that this option brings some risk with it as it essentially bypasses apt-secure. Reviewing the details in man apt-get informs us that it's possible to reduce that risk somewhat by additionally specifying one of the specialty options - in this case the suite label as that is the label that has been changed in buster, from stable to oldstable:
$ sudo apt-get --allow-releaseinfo-change-suite update
You can try apt instead of apt-get. Some sources will state without reservation that using apt instead of apt-get will simply make this issue go away - i.e.:
$ sudo apt update
# as opposed to:
$ sudo apt-get update
I won't dispute that - all I can say is that on my systems (RPi OS Lite), I have not been able to verify this. I get no prompt from sudo apt update asking me to approve anything. AIUI, apt depends upon /etc/apt/preferences and/or /etc/apt/preferences.d to relieve the user from making decisions - but these files are absent on my system. Debian's man page for apt_preferences states:
Preferences are a strong power in the hands of a system administrator but they can become also their biggest nightmare if used without care! APT will not question the preferences, so wrong settings can lead to uninstallable packages or wrong decisions while upgrading packages. Even more problems will arise if multiple distribution releases are mixed without a good understanding of the following paragraphs.
That's enough information for me, thank you. But seriously, this may work beautifully for some - using apt instead of apt-get may work for you. It's certainly the easiest solution here.
I don't presently know of other specific options within apt-get for doing this, but I'd have to guess there are as APT in general, and apt-get in particular is, uh... "full-featured" if nothing else. I also don't know what RPi's MO is for retaining the suite label beyond the official release date for RPi's version of bullseye. I'll update if I learn of their plans.
bustertobullseye. However,bullseyeis not yet in official release for the RPi, and it's unclear (to me) why the maintainers have made these repo changes without also making an announcement. Then again, the last announcement in that forum was in Aug 2020 :( But what to do? The answer below may be correct, but as it amounts to a security bypass, I'll wait on more clarity before taking that step.